The sensitivity of mammalian muscle spindle receptors is controlled by a complex arrangement of efferent innervation. These fusimotor fibers consist of collaterals of the alpha motor nerves in addition to the well-known gamma efferents. These "skeletofusimotor" fibers are known as beta axons. The proposed study will determine the effects of stimulating beta axons on the sensitivity of the muscle spindle primary endings to sinusoidal muscle stretch of small amplitudes (10-100 micron) in the cat under pentobarbital anesthesia. Physiological range of frequencies (0.1-20 Hz) will be employed in studies of input-output relationships and Bode plots to document the frequency response of the mammalian muscle spindles under beta axon control. Recordings from primary (Ia) muscle spindle afferents will be made in isolated dorsal root filaments while beta axons will be isolated in ventral rootlets for stimulation. A differential control of extrafusal-intrafusal contractions to beta axon stimulation will be obtained through intravenous administration of the drug dantrolene sodium which interferes with the excitation-contraction coupling in the extrafusal muscle. The effects of such loading of the muscle on the control of spindle sensitivity by beta axons will be determined.